Dental restoration materials, such as crowns, are commonly prepared through a precision casting technique. After an impression is made of a patient's teeth, the impression is sent to a laboratory where an exact wax replica is made of the tooth or portion of a tooth in need of replacement. The metal portion of the crown is formed by a lost wax process in which a mold is formed around the wax tooth replica, the wax is melted out, and the mold is then used to cast a replacement tooth.
First, the wax tooth replica is sprued and placed in a metal ring or removable plastic ring. An investment powder, made up of a binding system and a filler system, as well as some other chemicals, is then mixed with colloidal silica liquid to produce a slurry. This slurry is then poured into the ring and is allowed to set for one to two hours until it hardens into a mold, surrounding the wax tooth replica. (The slurry may alternatively be poured into an impression for forming a refractory die.) The mold is then placed in a furnace, and the furnace temperature is raised to 650.degree. C. or higher. This heating process is often referred to as burning-out the mold. During burn-out, the mold is subjected to high heat to melt the wax/plastic pattern and to complete the pyrolytic reactions.
After the wax pattern melts away, a hollow cavity is left. A molten metal, typically a precious or nonprecious metal alloy, is then poured into the cavity and allowed to cool. After the metal casting is removed from the impression, dental porcelains or alloy materials may then be sintered onto the crown or other dental restoration device for aesthetic effect to complete the process.
As was previously mentioned, the refractory powders used in this process contain a filler system and a binder system; some also contain small amounts of chemicals and wetting agents to regulate various desired properties of the system. A typical filler system may include one or more silica allotropes, such as quartz, tridymite, or cristobalite, in combination with various metallic oxides, such as aluminum oxide, calcium or aluminum silicates, and other materials, including zircon, zirconia, leucite, calcium fluoride, and pyrophylites. Normally, the filler system comprises 50 to 80 weight percent of the refractory powder. The binding system usually is made up of one or more different ammonium phosphate compounds, such as monobasic, dibasic, or hemi-basic ammonium phosphate, and magnesium oxide. Normally, the binder system comprises 20 to 50 weight percent of the refractory powder. The powder is mixed with a colloidal silica liquid to produce the slurry, whereupon the lost wax process is performed.
A problem encountered during the lost wax process is the breaking or cracking of the mold during the heating process. Explosive cracking may occur shortly after the mold is placed in a preheated furnace, typically during the first fifteen minutes. If explosive cracking occurs, the entire process must be conducted again from the beginning, at great cost. Even if explosive cracking does not occur, the mold material may fall off in chunks, or may spall, again resulting in a useless product. Micro-cracking, which may or may not result in a useless product but often yields a substandard one, may occur as the mold is heated above 750.degree. C.
Explosive cracking and spalling occur because of two concurrent problems. First, the binder system is decomposing at lower temperatures, thereby releasing the more volatile contents such as water and ammonia, causing a contraction of the system. Second, an uneven expansion of the system is caused by the temperature differential across the mold as it is heated, since the outer edges of the mold are at a higher temperature than the inner part of the mold. This occurs because the filler and binder ingredients contained in refractory powder materials are poor conductors of heat, especially at temperatures below 500.degree. C.
Therefore, in the prior art, refractory materials must be introduced into a furnace at very low temperatures, and the rate of heat increase must be kept low to avoid exploding the mold during the heating process. It is then necessary to cool the furnace down to room temperature before the next batch of molds can be introduced into the furnace.
In a typical process, in order to avoid explosive cracking, the mold is placed in a furnace at room temperature and is then gradually heated until a designated maximum temperature is reached, which usually takes from one to two hours, with an additional thirty minute holding period to allow the mold to soak in the heat. Thus, it takes approximately three hours for the furnace to complete the process of heating up and cooling down to make a single batch of products. This is very time-consuming and wastes a lot of energy, because all the energy used to heat the furnace is lost before the next batch can be put into the furnace. The repeated heating and cooling of the furnace also diminishes the furnace life, as it expands and contracts with each heating and cooling.